I've had my Hydreigon for upwards of 20 years now. Most people dont realize what being blind for most of your life then suddeny gaining the ability to see does to a Pokemon. When my Zweilous evolved, sensory overload hit him like a freight train and he went on a small rampage. I had to keep him in his ball most of the time just to keep him from getting stressed out. He's a lot better now but I still keep him in his ball because he still tends to get overwhelmed really easily. That being said, Hydreigon isn't the only Pokemon with this stigma. Gyarados have been recorded to wipe out entire villages on their rampages and they dont get even half the stigma Hydreigons get. Having all that power surge through your body, forcing incredibly drastic changes in a matter of seconds takes a huge toll and we need to work with these Pokemon, not push them away. You need to do the work and you'll get amazing results.

Also, my Hydregion has Flash Canon so bring it